Help to integrate custom star map poster designer with Printful API through Shopify platform

I need help in integrating my custom star map poster designer landing page [login to view URL] with the printful printing API [login to view URL] through my Shopify site [login to view URL] Items needed to be completed: 1. User flow for the API: A. User creates a map at [login to view URL] B. User submits map to be designed and is put in the Shopify cart C. User completes shipping and billing info in Shopify D. Shopify sends custom poster design and shipping details to Printful to be created and sent to the customer by Printul Step D is where I need your help 2. Need to make Star map poster consistent aspect ratio and DPI on screen and in PHP database download for upload to Printful. Need assistance in setting up final option for users to pay for a PDF/PNG download of their own. This process is 50% complete. Let me know if you can help!